Hitherto, in electrolytic refining of copper, during electrolytic refining using copper sulfate, contents of particularly silver (Ag) and S cannot be reduced, and it is difficult to obtain high-purity electrolytic copper having a purity of 5N (99.999%) or higher. Therefore, electrolytic refining using copper nitrate is performed (for example, Patent Document 1). In addition, it is known that a bath temperature is temporarily lowered and electrolytic refining is performed in two stages so as to reduce a content of impurities (for example, Patent Document 2). Moreover, it is also known that polyethylene glycol (PEG) or polyvinyl alcohol (PVA) is used as an additive so as to further reduce the contents of Ag and S (for example, Patent Document 3), and the PEG and PVA are synthetic polymer additives which do not include S and are stable, and the PEG and PVA include low contents of impurities (for example, Patent Document 3).
Recently, in the case where the high-purity electrolytic copper is used as a bonding wire, a concentration of impurities, particularly a content of S is the cause of wire fracture. Therefore, there is a strong demand for a reduction in the content of S.
However, in the electrolytic refining using copper nitrate as disclosed in J Patent Document 1, there is a problem in that the content of S can be reduced to only about 0.05 ppm. In addition, in the method of performing electrolytic refining in the two stages as disclosed in Patent Document 2, refining needs to be performed through electrolysis in the two stages while temporarily reducing the bath temperature to 10° C. or less and removing impurities using a filter. Therefore, there is a problem in that facility cost becomes high.
In the method of using PEG or PVA which does not contain S as an additive as disclosed in Patent Document 3, the content of S in the deposited high-purity electrolytic copper can be reduced to 0.005 ppm or less; and therefore, quality can be improved.
However, for example, in the case where PEG 1000 and the PVA 500 (1000 and 500 represent molecular weights) are used, there is no problem when a small cathode (SUS plate) which is a square where a length of each side is less than 30 cm (the area is less than 900 cm2) is used. However, when electrolysis is performed by using a large cathode (SUS plate) which is a square where a length of each side is 30 cm or more (the area is 900 cm2 or more), a phenomenon occurs in which high-purity electrolytic copper deposited on the cathode becomes very, brittle. Therefore, the deposited high-purity electrolytic copper is broken when being peeled off from the SUS plate. As a result, the yield of the high-purity electrolytic copper which proceeds to casting as the subsequent process is degraded. Therefore, there is a problem in that the productivity of high-purity electrolytic copper as the end product is greatly reduced.
On the other hand, when the molecular weight of the additive is increased (a molecular weight of PEG is in a range of 2000 or more) the brittleness is improved; however, a tensile stress is generated in the cathode (high-purity electrolytic copper) during the electrolysis due to the increase in the molecular weight. When the tensile stress is increased, the cathode (high-purity electrolytic copper) warps and is peeled off from the SUS plate during the electrolysis. Even in this phenomenon, in the case where the small cathode (SUS plate) which is a square where a length of each side is less than 30 cm (the area is less than 900 cm2) is used and the electrolysis time is short, the cathode (high-purity electrolytic copper) is rarely peeled off although the cathode warps. Therefore, there is no particular problem. However, in the case of mass production, it is essential to perform electrolysis at a current density as high as possible using a cathode having a large area. Under this condition, there is a problem in that high-purity electrolytic copper deposited on the cathode is easily peeled off, and the high-purity electrolytic copper is peeled off from the cathode plate and falls into an electrolytic cell during the electrolysis.